Heretofore, in polishing or grinding metal sheets or plates, the sheets have been relatively narrow and, consequently, the belts, rolls and mounting assemblies have been correspondingly narrow. In such prior assemblies, it was recognized that there be belt tensioning means and also that the belt be mounted on an oscillating idle or tension roll, with the oscillation controlled to effect the oscillatory movement of the endless abrasive belt. Such prior art devices are disclosed in U.S. Pat. No. 2,597,256 and U.S. Pat. No. 3,665,650. The belts and rolls of such patents may be up to approximately 72" long maximum with each roll assembly, including roll and mounting frames or brackets, weighing about 3000 pounds.